1. Field of the Invention
The present invention relates to a photographic film cassette with a flanged spool about which a filmstrip is coiled, and in particular to a film cassette capable of automatically advancing a non-protruding film leader to the outside of the cassette shell responsive to unwinding rotation of the spool. More specifically, the present invention relates to a photographic film cassette in which flanges of the spool have optimum elasticity, durability and film advancing power.
2. Background Art
Recently, film cassettes have been proposed which can be operated to automatically advance a film leader out of the cassette shell by rotating the film spool in the unwinding direction, for example, as disclosed in U.S. Pat. Nos. 4,834,306 and 4,832,275 and EP 0406815 A2. The film leader is non-protruding, i.e. it is located entirely within the cassette shell unless the film cassette is loaded in a camera, film inspector or the like. Therefore, this type of film cassette is convenient and safer to handle both for the user and for photofinisher.
Major elements of the leader-advancing type film cassette such as the cassette shell, the spool, the flanges are formed by resin molding.
The flanges are provided for tightly and neatly winding the filmstrip in a roll on the spool and preventing loosening of the film roll. For example, the flanges are formed as discs having circumferential lips. The respective lips cover lateral sides of the outermost convolution of the film roll to confine it in the radial direction while the flanges confine the film roll in the axial direction.
A separating claw and constraining ribs are formed inside the cassette shell. The separating claw is adapted to peel the film leader, i.e., the end of the outermost convolution of the film roll in the vicinity of a film passage mouth. The constraining ribs constrain the flanges toward the film roll so as to keep the flanges in contact with end faces of the film roll. The constraining ribs opposing to each other in the vicinity of the film passage mouth are spaced from each other a distance to allow the flange, to release the confinement of the film roll so the filmstrip may be unwound and advanced out of the cassette shell after the film leader is peeled by the separating claw.
The above-described members of the film cassette are conventionally formed from high impact polystyrene (HIPS) resin or high density polyethylene resin because of their superior moldability. However, the above-mentioned materials are insufficient for use in practice. Especially for the flanges, because of rubbing against the constraining ribs and the edges of the filmstrip while the spool rotates, the above-mentioned materials are too weak. The flange would quickly be worn out by friction and produce resin particles. The resin particles travel on the filmstrip and damage the camera, the photographic properties of the filmstrip, or images on the filmstrip.
The friction of the flanges and the amount of particles generated are dependent upon distances between the constraining ribs of opposite sides, especially those disposed in the vicinity of the film passage mouth. The distances between the opposing ribs also have influence on film advancing power and durability of the flanges. If the distance is too large, the flanges cannot securely hold or confine the film roll enough to smoothly advance the filmstrip. If the distances are too small, the friction becomes too large and the durability is lowered.
For reliable advance of the filmstrip to the outside, it is desirable to curve the filmstrip in a portion which is guided to the film passage mouth, in the lateral direction of the filmstrip. For this purpose, the lips of the flanges push the lateral edges of the filmstrip toward each other. If the flanges are too soft, the filmstrip will not curve sufficiently. If the flanges are too hard, the filmstrip will curve so much that the filmstrip interferes with the film passage mouth, a film guide surface of the camera, or the like. Also, the friction between the flanges and the filmstrip will increase so much that lowers the durability of the flanges and causes remarkable noise.
Also the flexibility of the filmstrip has influence on film advancing power and durability of the flanges. The film flexibility depends upon the base material, the thickness, the type of the photosensitive emulsion layer, and so forth. Also, the film flexibility varies depending upon the ambient temperature and humidity.
Thermoplastic deformation of the flanges is another problem. In order to make the flanges elastic, the thickness of the flanges should be small. Also for the compactness of the film cassette, thin flanges are desirable. However, the film unit can be left in high temperature circumstances, for example, in a car staying outdoor in summer where the ambient temperature may reach 80.degree. C. The flange made of the above-mentioned material would be thermally deformed in that case.
To solve the above problems, a film cassette has been proposed by the present Applicant, wherein as a material of the flanges, styrene-synthetic rubber copolymer is mixed with modified polyphenylene at a mixture ratio from 3:7 to 8:2.
Indeed this material can improve the durability of the flanges to a certain extent, but the durability is insufficient in view of the expected service conditions of the leader-advancing type film cassette where the filmstrip will be repeatedly withdrawn from and wound into the cassette shell, for example, for exposure, development, inspection, and printing.